Question about new 50 Mask mode

[gpvillamil]

I have a question about the new 50 Mask mode - it looks handy, and close to what I need but not exactly.

What I'd like is for one video to fill in the white area of the mask, and another to fill in the black area. Right now, I can *almost* do it by setting up a layer with my video masks, setting the blend mode to 50 Mask at 100% opacity, having the "white" channel below it. The black channel is on a layer above, with the transfer mode set to Lighten. This almost works, except that sometimes particularly light parts of the "black" channel will overlay darker parts of the "white" channel.

Is there something I'm not getting?

Also, how is 50 Mask mode different from Multiply at 50% opacity?